Durability and Mechanical Properties of Concrete Reinforced with Basalt Fiber-Reinforced Polymer (BFRP) Bars: Towards Sustainable Infrastructure

polymers

Review Durability and Mechanical Properties of Concrete Reinforced with Basalt Fiber-Reinforced Polymer (BFRP) Bars: Towards Sustainable Infrastructure

Osama Ahmed Mohamed 1,* , Waddah Al Hawat 1 and Mohammad Keshawarz 2

1 Departent of Civil Engineering, College of Engineering, Abu Dhabi University, Abu Dhabi PO Box 59911, United Arab Emirates; [email protected] 2 Department of Civil & Biomedical Engineering, College of Engineering, Technology, and Architecture, University of Hartford, West Hartford, CT 06117, USA; [email protected] * Correspondence: [email protected]

Abstract: Reducing the fingerprint of infrastructure has become and is likely to continue to be at the forefront of stakeholders’ interests, including engineers and researchers. It necessary that future buildings produce minimal environmental impact during construction and remain durable for as long as practicably possible. The use of basalt fiber-reinforced polymer (BFRP) bars as a replacement for carbon steel is reviewed in this article by examining the literature from the past two decades with an emphasis on flexural strength, serviceability, and durability. The provisions of selected design and construction guides for flexural members are presented, compared, and discussed. The bond of BFRP bars to the surrounding concrete was reportedly superior to carbon steel when BFRP was helically wrapped and sand coated. Experimental studies confirmed that a bond coefficient kb = 0.8, Citation: Mohamed, O.A.; Al Hawat, which is superior to carbon steel, may be assumed for sand-coated BFRP ribbed bars that are helically W.; Keshawarz, M. Durability and wrapped, as opposed to the conservative value of 1.4 suggested by ACI440.1R-15. Code-based models Mechanical Properties of Concrete overestimate the cracking load for BFRP-reinforced beams, but they underestimate the ultimate load. Reinforced with Basalt Exposure to an alkaline environment at temperatures as high as 60 ◦C caused a limited reduction in Fiber-Reinforced Polymer (BFRP) bond strength of BFRP. The durability of BFRP bars is influenced by the type of resin and sizing used Bars: Towards Sustainable to produce the bars. Infrastructure. Polymers 2021, 13, 1402. https://doi.org/10.3390/ Keywords: basalt fiber-reinforced polymers; concrete; reinforcing bars; sustainable construction; polym13091402 durability; bond to concrete

Academic Editor: Farid Abed

Received: 27 March 2021 Accepted: 20 April 2021 1. Introduction Published: 26 April 2021 Sustainability and durability of building structures are amongst the leading design criteria for new infrastructure. The contribution of the production of cement to the emission Publisher’s Note: MDPI stays neutral of CO2 and environmental pollution prompted the pursuit of alternative cementitious ma- with regard to jurisdictional claims in terials, including, but not limited to, fly ash and slag. Partial replacement of cement with fly published maps and institutional affil- ash, slag, and/or silica fume has become a common practice and the mechanical properties iations. of such concrete types have been studied extensively. On the other hand, exclusive use of materials such as fly ash and slag as sole cementitious materials activated using carefully selected alkalis has attracted the attention of researchers. Similarly, traditional reinforcing steel bars, despite their favorable mechanical properties, are associated with significant Copyright: © 2021 by the authors. emission of CO2 during the manufacturing process, not to mention corrosion that may lead Licensee MDPI, Basel, Switzerland. to a loss of cross-section. In the past two decades, interest has been renewed in reinforcing This article is an open access article bars made of fiber-reinforced polymers (FRPs). The unidirectional fibers that constitute distributed under the terms and typically more than 75% by volume are made of glass, carbon, aramid, and, more recently, conditions of the Creative Commons basalt. Basalt FRP (BFRP) reinforcing steel bars offer numerous favorable properties over Attribution (CC BY) license (https:// traditional reinforcing steel bars, including, but not limited to, high tensile strength, corro- creativecommons.org/licenses/by/ sion resistance, and nonmagnetic nature. Studies by [1] concluded that BFRP-reinforced 4.0/).

Polymers 2021, 13, 1402. https://doi.org/10.3390/polym13091402 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1402 2 of 23

beams have similar global warming potential (GWP) compared to cast-in-place concrete reinforced with 100% recycled carbon steel and, as a result, BFRP-reinforced beams have a limited environmental advantage compared to steel-reinforced concrete beams. On the other hand, concrete beams reinforced with BFRP prestressing bars have much lower GWP compared to beams prestressed with steel bars. In aggressive environments, traditional reinforcing steel bars are susceptible to corrosion that could influence the structure0s life span. Marine environments and parts of the world where deicing salts must come in contact with concrete are examples of situations where traditional reinforcing steel bars may be subjected to corrosion. It is often less expensive and environmentally friendly to use sea sand for concrete structures that will be in contact with seawater, in which case the noncorroding FRP reinforcing bars would be a durable alternative to traditional steel. The magnetic properties of FRP bars make them suitable for consideration in structural elements surrounding magnetic resonance imaging (MRI) units and any other equipment sensitive to magnetic fields. Sea sand typically contains chloride irons that may cause corrosion of reinforcing steel bars. Design guides for concrete reinforced with FRP bars, such as ACI440.1R-15 [2], were developed and are continuously updated. Most design guides explicitly refer to FRP bars made of glass (GFRP), carbon (CFRP), or aramid (AFRP) fibers, which were studied extensively. However, ACI440.1R makes no explicit reference to concrete-reinforcing FRP bars made of basalt fibers. BFRP bars offer many favorable properties such as high temperature resistance and favorable behavior in an acidic environment, in addition to ease of manufacturing. BFRP reinforcing bars typically fall between CFRP and GFRP bars in terms of strength and stiffness. Studies have shown that GFRP reinforcing bars can be used effectively as corrosion-resistant reinforcement for hollow concrete columns (HCCs) that have many applications, such as bridge piers [3]. Glass fibers were also used successfully in nonbuilding structures such as composite sleepers for railway tracks [4]. GFRP reinforcing bars are susceptible to simulated alkaline environments, resulting in degrading of the fiber–matrix interface [5]. The purpose of this article is to provide a critical review of the literature on the mechanical properties and durability of concrete reinforced with FRP bars made with the promising basalt fibers. This article presents, to practicing engineers, the current state of knowledge on the properties BFRP-reinforced flexural members in support of decision making related to selecting materials for engineering projects. In addition, the article explores the merits and demerits of BFRP in comparison to some of the existing alternatives (e.g., GFRP and CFRP) in terms of strength and durability. Research needed to fill gaps in the knowledge in terms of the durability and strength of BFRP reinforcing bars is identified in this article as well.

2. Composition and Properties of Basalt Fiber-Reinforced Polymers (BFRPs) A basalt FRP bar is a composite material consisting of rigid polymer resin bounding unidirectional basalt fibers. Basalt fibers are produced by melting queried and crushed natural volcanic basalt rocks at a temperature of nearly 1400 ◦C[6]. The molten rock is extruded through small nozzles to produce continuous filaments of basalt fibers ranging in diameter from 13 to 20 µm. A critical process in the manufacturing of fibers, in general, is known as fiber sizing. Sizing involves the application of a thin layer of mainly organic material known as the size to the surface of the fiber. Most importantly, the short-term and long-term performance of FRP bars is critically influenced by the optimization of the fiber sizing as well as the fiber–matrix interface [7]. The fiber sizing film consists of a film former and a coupling agent. The film former protects, lubricates, and holds the fibers together while ensuring their separation when the fibers come in contact with the resin. The coupling agent, typically an alkoxysilane compound, serves to bond the fibers to the matrix resin [8]. However, the composition and process of applying the fiber size layer vary significantly amongst manufacturers, resulting in variations in properties of FRP bars made of the same type of fiber and sometimes the same resin type. Polymers 2021,Polymers13, 1402 2021, 13, x FOR PEER REVIEW 3 of 23 3 of 24

The resultingThe composite resulting material,composite consisting material, ofconsisting polymeric of resinpolymeric and fibers, resin and offers fibers, nu- offers numerous favorablemerous properties, favorable including,properties, but including, not limited butto, not high limited tensile to, strength,high tensile with strength, appli- with ap- cations in buildingplications new in structures, building such new as FRPstructures, reinforcing such bars, as orFRP retrofitting/strengthening reinforcing bars, or retrofit- deficient existingting/strengthening structures using deficient FRP sheetsexisting and/or structures strips using [9]. FRP sheets and/or strips [9]. BFRP barsBFRP are commonly bars are commonly manufactured manufactured through the thro pultrusionugh the pultrusion process, which process, in- which involves pullingvolves the pulling continuous the continuous fibers through fibers athrough die that a die is circular that is circular in cross-section in cross-section and and con- contains resin.tains Theresin. FRP The bars FRP are bars formed are formed once theonce resin the curesresin (thermosets)cures (thermosets) in the in die. the die. The The amountamount of basalt of basalt fiber in fiber BFRP in barsBFRP is bars not standardized,is not standardized, but the but fiber the content fiber content most most fre- frequentlyquently reported reported in the literature in the literature falls in falls the rangein the 75%range to 75% 90% to [10 90%,11]. [10,11]. Automated Automated wet- wet-layuplayup is another is another method method to manufacture to manufacture BFRP BFRP bars bars that reportedlythat reportedly offers offers the samethe same degree degree of variationof variation in mechanicalin mechanical properties properties as as the the pultrusion pultrusion process process [12 [12].]. As As the the resin resin has much has much lowerlower strength strength compared compared to to the the fibers, fibers, the the tensile tensile strength strength and and stiffness stiffness of of BFRP BFRP bars var- bars variesies depending depending on theon overallthe overall volume volume of fibers of tofibers volume to volume of FRP. Vinylof FRP. ester Vinyl and ester and isophthalicisophthalic polyester are polyester common are types common of resin types matrix of resin used matrix to manufacture used to manufacture BFRP. BFRP. FRP bars areFRP more bars sensitive are more to sensitive fire than to steel fire bars.than However,steel bars. becauseHowever, the because FRP bars the FRP bars are embeddedare embedded in concrete, in they concrete, do not they contribute do not contribute to fire severity to fire nor severity toxicity. nor Nonetheless, toxicity. Nonetheless, FRP-reinforcedFRP-reinforced concrete elements concrete have elements lower resistance have lower to fire resistance compared to tofire steel-reinforced compared to steel-rein- concrete elements [13]. More importantly, at temperatures close to the glass transition forced concrete elements [13]. More importantly, at temperatures close to the glass transi- temperature of the polymer, Tg, mechanical properties of the polymer deteriorate, and its tion temperature of the polymer, Tg, mechanical properties of the polymer deteriorate, ability to transfer stresses between the fiber and the surrounding concrete decreases [14]. and its ability to transfer stresses between the fiber and the surrounding concrete de- The structural implication is the degrading of the bond strength between FRP bars and creases [14]. The structural implication is the degrading of the bond strength between FRP concrete. Glass transition temperatures for most resins used to manufacture FRP reinforced bars and concrete. Glass transition temperatures for most resins used to manufacture FRP bars range from 93 ◦C to 120 ◦C. reinforced bars range from 93 °C to 120 °C. BFRP bars may be 2.3 times stronger or more , in terms of ultimate strength (fu), BFRP bars may be 2.3 times stronger or more , in terms of ultimate strength (fu), than than traditional steel reinforcing. However, the modulus of elasticity of traditional steel traditional steel reinforcing. However, the modulus of elasticity of traditional steel may be may be 3.5 times or greater than BFRP. BFRP elastic moduli varying from 44.5 to 71 MPa 3.5 times or greater than BFRP. BFRP elastic moduli varying from 44.5 to 71 MPa were re- were reported in the literature [11,15,16], depending on resin type, manufacturer, and sometimesported bar diameter. in the literature Unlike traditional [11,15,16], carbon dependin steel,g on FRP resin bars type, do not manufacturer, exhibit yielding, and sometimes as shown inbar Figure diameter.1. Tensile Unlike strengthtraditional reported carbon steel, in the FR literatureP bars do variednot exhibit from yielding, 1100 to as shown in 1565 [11,15Figure,16]. These 1. Tensile wide strength ranges reported of values in the for lit theerature tensile varied strength from 1100 and to modulus 1565 [11,15,16]. of These elasticity werewide reported ranges of for values BFRP for produced the tensile by strength different and manufacturers, modulus of elasticity which not were only reported for reflect variationBFRP inproduced the properties by different of resin manufacturers, but also manufacturing. which not only Nonetheless, reflect variation variability in the proper- in moduli andties strengthof resin but were also reported manufacturing. in BFRP barsNonetheless, produced variability by the same in moduli manufacturer, and strength were although withreported less dispersion. in BFRP bars In produced comparison, by duethe same the homogeneity manufacturer, of although steel, the with modulus less dispersion. of elasticityIn of comparison, and tensile strengthdue the canhomogeneity largely be assumedof steel, the to be modulus constant of for elasticity all practical of and tensile purposes. strength can largely be assumed to be constant for all practical purposes.

Figure 1. Typical stress–strain relationship of carbon steel and basalt ﬁber-reinforced polymer Figure 1. Typical stress–strain relationship of carbon steel and basalt fiber-reinforced polymer (BFRP) bars. (BFRP) bars.

Polymers 2021, 13, x FOR PEER REVIEW 4 of 24

It is to be expected that the external surface configuration of BFRP bars affects the effectiveness of bonds to the surrounding concrete. The external surface may be helically wrapped with fibers, as shown in Figure 2, with or without additional sand coating. BFRP bars may also have deformations (ribs or indents) without helical fiber wrapping, with or

Polymers 2021, 13, 1402 without sand coating. The most common are ribbed BFRP bars with helical fiber wrapping4 of 23 and sand coating. Kevlar fibers (0.4 mm in diameter) are often used for helical wrapping [17]. Figure 2 shows schematics of various FRP bar configurations. Ribbed and helically wrapped BFRP bars that are sand coated provide the highest bond strength, as will be discussedIt is to later be expected in this article. that the Nonetheless, external surface the method configuration of sand of coating BFRP bars appears affects to thealso effectivenessaffect the bond of bonds strength to the although surrounding no standardized concrete. The method external of sand surface coating may beis helicallyavailable. wrappedSand coating with of fibers, FRP reinforcing as shown bars in Figure was used2, with before or withoutthe advent additional of BFRP sandbars to coating. enhance BFRPbond barsstrength may alsoand havewas proven deformations to enhance (ribs bond or indents) strength without [18]. It helical is typical fiber to wrapping, apply the withhelically or without wound sand fibers coating. and sand The coating most common after the are pultrusion ribbed BFRP process, bars but with before helical the fiber ther- wrappingmosetting and of the sand polymeric coating. resin Kevlar [19]. fibers (0.4 mm in diameter) are often used for helical wrappingAs a [ 17result]. Figure of a 2lack shows of standardization, schematics of various the increase FRP bar in configurations. BFRP bar area Ribbed due to andsand helicallycoating is wrapped inconsistent BFRP and bars may that vary are with sand ba coatedr diameter, provide even the when highest produced bond strength, by the same as willmanufacturer be discussed [20]. later Similarly, in this article.tensile strength Nonetheless, and modulus the method of elasticity of sand coatingalso varied appears from toone also bar affect size to the another bond strengthin BFRP obtained although fr noom standardized the same source. method For wrapped of sand coating BFRP bars is available.without sand Sand coating, coating ofvariations FRP reinforcing in bond barsstrength was usedwere beforealso reported the advent based of BFRPon rib bars size toand enhance rib spacing bond [21], strength but further and was studies proven are to needed enhance to quantify bond strength the observation. [18]. It is typicalBFRP bars to applywith thewoven helically surfaces wound (no ribs) fibers are and manufacturing sand coating afterfor use the as pultrusion prestressing process, bars, butwith before lower thebond thermosetting strength compared of the polymeric to ribbed resin bars [[22].19].

FigureFigure 2. 2. Fiber-reinforced polymers polymers (FRP (FRP)) bar bar surface surface configurations: conﬁgurations: (a) (Ribbeda) Ribbed and and helically helically wrapped,wrapped, (b ()b ribbed,) ribbed, helically helically wrapped, wrapped, and and sand sand coated, coated, (c ()c indented,) indented, (d ()d ribbed.) ribbed.

3. BondAs a of result BFRP of Bars a lack to ofConcrete standardization, the increase in BFRP bar area due to sand coatingThe is inconsistentability of concrete and may flexural vary members with bar diameter,to sustain even applied when loads produced is highly by dependent the same manufactureron the bond characteristics [20]. Similarly, between tensile strength basalt fiber-reinforced and modulus of polymer elasticity (BFRP) also varied and the from sur- onerounding bar size concrete. to another Specifications in BFRP obtained for fromBFRP the bars same are source. not Forexplicitly wrapped mentioned BFRP bars in withoutACI440.1R-15 sand coating, [2]. However, variations CSA in bond[23] specif strengthies werea minimum also reported bond basedstrength on of rib 8 size MPa and for rib spacing [21], but further studies are needed to quantify the observation. BFRP bars with woven surfaces (no ribs) are manufacturing for use as prestressing bars, with lower bond strength compared to ribbed bars [22].

3. Bond of BFRP Bars to Concrete The ability of concrete flexural members to sustain applied loads is highly dependent on the bond characteristics between basalt fiber-reinforced polymer (BFRP) and the surrounding concrete. Specifications for BFRP bars are not explicitly mentioned in ACI440.1R-15 [2]. However, CSA [23] specifies a minimum bond strength of 8 MPa for Polymers 2021, 13, 1402 5 of 23

BFRP bars. Much higher bond strength was reported in the literature for most BFRP bar surface configurations/preparations. Henin et al. [24] noted that fatigue loads decrease bond stiffness and increase slip for different bar–rib configurations, compared to static loads. However, the phenomenon depends on the fatigue stress level. The higher the fatigue stress level, the higher the bond stiffness. The investigators noted a slight improvement in bond strength under fatigue load compared to a static load. On the other hand, fatigue load aggravates the bond interface damage. Bond characteristics reported in the literature include studies on different FRP bar configurations such as ribbed/indented, ribbed/indented/helically wrapped without sand coating, and ribbed/indented/helically wrapped but without sand coating. The optimum bond strength was obtained when BFRP bars were ribbed and sand coated along the bar length [22]. The effect of combined helical wrapping and sand coating reportedly produces the best bond characteristics, except for the study by Solym and Balazs [22], which reported sand coating of ribbed bars to produce better bond characteristics than combining sand coating and helical wrapping of bars. Several guides, standards, and codes developed test procedures to determine bond strength that was used to develop bond–slip relationships [23,25,26]. The bar pull-out test is the most common amongst all standards and guides. The bond strength, τ (MPa, psi), is related to the tension force, F (N, Ibf), through Equation (1). F τ = (1) Cb l where: τ = average bond stress, MPa (psi); F = tensile force, N (lbf);

Cb = effective circumference of FRP bar, calculated as 3.1416 db where db is the effective bar diameter of the bar, calculated according to Test Method D7250/D7250M, mm (in); l = bonded length, mm (in). Typically, the failure load is inﬂuenced by the bond length, l, which is deﬁned as the length in contact with concrete.

3.1. Effect of Rib Spacing and Rib Depth of BFRP Reinforcing Bars on Bond Strength Studies by [21] concluded that rib spacing and rib depth of BFRP bars affect the bond strength. Figure3 shows a bond–slip relationship for 10 mm BFRP bars with rib spacing of 6, 8, 10, and 12, and rib depth of 0 mm, 0.5 mm, or 1 mm (RS8RD1 means rib spacing of 8 mm and rib depth of 1 mm). Generally speaking, for bars with the same rib spacing (e.g., RS10), the larger the rib depth, the higher the bond strength (RS10RD1 exhibited higher strength compared to RS10RD0.5 and RS10RD0). There are three mechanisms for the bond between BFRP bars and concrete, namely, chemical adhesion, mechanical interlocking, and friction. Friction bonding is particularly signiﬁcant when BFRP bars are coated with sand [11]. For loads up to 20% of the maximum load, the bond is mostly due to chemical adhesion, and there is very little movement at the load end and no displacement at the free end regardless of the rib spacing or rib depth [21]. Movement at the free end begins at roughly 40% of the maximum load and the strength–slip relationship becomes nonlinear. When the maximum load is reached, shear fracture occurs at the interface between the BFRP and surrounding concrete. Polymers 2021, 13, 1402 6 of 23 Polymers 2021, 13, x FOR PEER REVIEW 6 of 24 Polymers 2021, 13, x FOR PEER REVIEW 6 of 24

2525

2020 RS12RD1 1515 RS12RD1 RS10RD0.5RS10RD0.5 10 10 RS10RD0RS10RD0 RS10RD1 5 RS10RD1 5 RS8RD1 Pull-Out Stress (MPa) RS8RD1 Pull-Out Stress (MPa) RS6RD1 00 RS6RD1 0123456701234567 Slip Displacement (mm) Slip Displacement (mm) Figure 3. Pull-out test result showing stress versus slip for 10 mm diameter BFRP bars with bond FigureFigure 3.3. Pull-outPull-out testtest result showing stress versus slip for for 10 10 mm mm diameter diameter BFRP BFRP bars bars with with bond bond length l = 5D with various rib spacings and rib depths. RS12RD1 = Rib spacing of 12 mm and rib lengthlength ll == 5D with various rib spacings spacings and and rib rib de depths.pths. RS12RD1 RS12RD1 = =Rib Rib spacing spacing of of 12 12 mm mm and and rib rib depth of 1 mm. Reprinted with permission from ref. [21]. Copyright 2021 ASTM International. depthdepth ofof 11 mm.mm. ReprintedReprinted withwith permissionpermission fromfromref. ref.[ 21[21].]. CopyrightCopyright2021 2021 ASTM ASTM International. International.

TheTheThe maximum maximummaximum bond bondbond strength strengthstrength increases increasesincreases with withwith an anan increase increaseincrease in inin bond bondbond length lengthlength for forfor bars barsbars of ofof thethethe same samesame material, material,material, diameter, diameter,diameter, rib ribrib spacing, spacing,spacing, and anandd rib ribrib depth. depth.depth. Figure FigureFigure4 4shows4 showsshows the thethe stress–slip stress–slipstress–slip relationshiprelationshiprelationship for forforl = ll 10D. == 10D.10D. Comparing ComparingComparing Figure FigureFigure4, it can 4,4, it beit can seen,can bebe for seen,seen, example, forfor example,example, that bond thatthat strength bondbond forstrengthstrength RS12RD1 for for RS12RD1 RS12RD1 was 21 MPa was was with2121 MPa MPal = with with 5D, whichll = = 5D, 5D, which iswhich less is thanis less less the than than strength the the strength strength of the of of same the the same same bar (33barbar MPa) (33 (33 MPa) MPa) when when whenl = 10D. l l = = 10D. 10D.

4040

RS12RD1 3030 RS12RD1 RS10RD0.5RS10RD0.5 RS10RD0 2020 RS10RD0 RS10RD1RS10RD1 RS8RD1 1010 RS8RD1 RS6RD1RS6RD1 Pull-Out Stress (MPa) Pull-Out Stress (MPa) 00 012345012345 SlipSlip Displacement Displacement (mm) (mm)

Figure 4. Pull-out stress showing stress versus slip for 10 mm diameter BFRP bars, with l = 10D Figure 4. Pull-out stress showing stress versus slip for 10 mm diameter BFRP bars, with l = 10D Figureand various 4. Pull-out rib spacings stress showing (RS) and stress rib depths versus (RD). slip for 10 mm diameter BFRP bars, with l = 10D and variousand various rib spacings rib spacings (RS) and(RS) rib and depths rib depths (RD). (RD).

3.2.3.2.3.2. BondBond Bond Strength–SlipStrength–Slip Strength–Slip RelationshipRelationship Relationship TheTheThe relationship relationshiprelationship between betweenbetween bondbondbond strength strengthstrength andandand slippage slippageslippage betweenbetweenbetween BFRPBFRPBFRP andandand thethethe sur-sur-sur- roundingroundingrounding concreteconcrete concrete isis is essentialessential essential toto to thethe the designdesign design ofof of flexuralflexural flexural members.members. members. AsAs As indicatedindicated indicated earlier,earlier, earlier, the the the effectivenesseffectivenesseffectiveness of ofof the thethe bond bondbond strength strengthstrength depends dependsdepends on onon several severalseveral factors, factors,factors, including, including,including, but butbut not notnot limited limitedlimited to,to,to, surface surfacesurface configuration/preparation configuration/preparationconfiguration/preparation ofofof the thethe BFRP BFRPBFRP bars. bars.bars. Several SeveralSeveral studies studiesstudies have havehave shown shownshown that thatthat ribbed/indented,ribbed/indented,ribbed/indented, helically helically wrapped, wrapped, andand sand-coatedsandsand-coated-coated BFRPBFRPBFRP providesprovidesprovides the thethe highest highesthighest bond bondbond strengthstrengthstrength compared comparedcompared to toto BFRP BFRPBFRP without withoutwithout wrapping wrappingwrapping or oror without withoutwithout sand sandsand coating coatingcoating [ [27].27[27].]. One OneOne study, study,study, however,however,however, reported reportedreported better betterbetter bond bondbond strength strengthstrength for forfor sand-coated sand-coatedsand-coated ribbed ribbedribbed FRP FRPFRP bars barsbarswithout withoutwithout helical helicalhelical wrappingwrappingwrapping compared comparedcompared to toto sand-coated sand-coatedsand-coated bars barsbars with withwith helical helicalhelical wrapping wrappingwrapping [22]. [22].[22]. It is It worthIt isis worthworth noting notingnoting that thethatthat latter thethe latter studylatter study showedstudy showedshowed a greater aa greatergreater variety variety ofvariety results ofof from resultsresults one fromfrom sample oneone to samplesample another toto and anotheranother samples andand weresamplessamples provided were were byprovidedprovided different by by manufacturers. different different manufactur manufactur The firsters.ers. phase The The offirst first the phase phase bond strength–slipofof the the bond bond strength– strength– relation isslipslip linear, relationrelation corresponding isis linear,linear, correspondingcorresponding to a small slippage, toto aa smsm whichallall slippage,slippage, is attributed whichwhich to isis chemical attributedattributed adhesion. toto chemicalchemical For wrappedadhesion.adhesion. BFRP For For wrapped wrapped bars with BFRP BFRP primary bars bars sand with with coating, primary primary the sand sand linear coating, coating, phase the the is followedlinear linear phase phase by a is is nonlinear followed followed phase up to the peak bond strength. No significant increase in bond strength occurs after byby aa nonlinearnonlinear phasephase upup toto thethe peakpeak bondbond strength.strength. NoNo significantsignificant increaseincrease inin bondbond the peak until complete slippage failure occurs (Figure5a). When wrapped BFRP bars are strengthstrength occursoccurs afterafter thethe peakpeak untiluntil completecomplete slippageslippage failurefailure occursoccurs (Figure(Figure 5a).5a). WhenWhen treated with a secondary sand coating, the peak bond strength is much larger than the wrappedwrapped BFRP BFRP bars bars are are treated treated with with a a secondary secondary sand sand coating, coating, the the peak peak bond bond strength strength is is

Polymers 2021, 13, 1402 7 of 23

Polymers 2021, 13, x FOR PEER REVIEW 7 of 24

bars with primarymuch sand larger coating, than as the shown bars in with Figure primary5b. The sand peak coa strengthting, as is followed shown in by Figure a 5b. The signiﬁcant droppea ink bondstrength strength is followed due to partialby a significant failure of thedrop secondary in bond sandstrength coating due layer. to partial failure of the secondary sand coating layer. The primary sand coating layer causes an increase The primary sandin bond coating strength layer causes again an (third increase phase), in bond as shown strength in again Figure (third 5b, phase), up to as a peak bond shown in Figurestrength5b, up which to a peak then bond remains strength essentially which then constant remains unti essentiallyl complete constant bond failure occurs until complete[27]. bond failure occurs [27].

16 14 12 10 8 6 4 Bond Strength (MPa) 2 0 0 0.5 1 1.5 2 2.5 Slip (mm) (a) 30

Bond Strength (MPa) 5

0 00.511.522.5 Slip (mm) (b)

Figure 5. BondFigure strength–slip 5. Bond relationship strength–slip for relationship 10-mm diameter for 10-mm wrapped diameter samples wrapped (each color samples is different (each sample) color is BFRP bars with (a) primarydifferent sand sample) coating,BFRP (b) secondary bars with sand (a) primary coating. sandReprinted coating, with (b )pe secondaryrmission from sand ref. coating. [27]. Copyright Reprinted 2021 Else- vier Science with& Technology permission Journals. from ref. [27]. Copyright 2021 Elsevier Science & Technology Journals.

When wrappedWhen BFRP wrapped reinforcing BFRP bars reinforcing are not coatedbars are with not sand,coated the with bond sand, strength the bond strength is is affected byaffected rib depth by (half rib depth of the (half difference of the difference between the between outer diameterthe outer includingdiameter including the the ribs ribs and innerand diameter inner excludingdiameter theexcluding rib), rib the spacing, rib), rib and spacing, rib conﬁguration and rib configuration (spacing and (spacing and depth/heightdepth/height affect the mechanical affect the interlock mechanical phase interlock of the totalphase bond of the strength). total bond Therefore, strength). Therefore, large rib spacinglarge (or rib lack spacing thereof) (or leads lack to thereof) a much leads weaker to bonda much strength weaker of BFRPbond barsstrength com- of BFRP bars pared to unribbedcompared bars, regardlessto unribbed of bars, sand regardless coating [21 of]. Thesand sand coating coating [21]. enhances The sand bond coating enhances strength throughbond a contributionstrength through to both a contribution the friction component to both the and friction enhancing component mechanical and enhancing me- interlocking. chanical interlocking.

3.3. Bond Coefﬁcient3.3. Bond and Coefficient Flexural Crack and ControlFlexural of Crack BFRP-Reinforced Control of BFRP-Reinforced Concrete Flexural Concrete Members Flexural Similar toMembers traditional carbon steel, the characteristics of the bond between FRP reinforcing bars and the surroundingSimilar to traditional concrete affects carbon crack steel, width the characteristics under service load.of the The bond current between FRP reinforcing bars and the surrounding concrete affects crack width under service load. The Polymers 2021, 13, 1402 8 of 23

philosophy for controlling service load crack width, w, in ACI318 was also adopted in ACI440.1R [2], which relies on controlling the spacing between the reinforcing bars. Equa- tion (2) expresses the maximum spacing between tension reinforcing FRP bars. Deﬁnitions of all parameters for the equations appearing in this article are listed in Abbreviations.

Ef w Ef w Smax = 1.15 − 2.5Cc ≤ 0.92 (2) f f skb f f skb

where:

Ef = design or guaranteed modulus of elasticity of FRP deﬁned as mean modulus of sample of test specimen (Ef = Eaverage), MPa (psi); ffs = stress level induced in FRP at service load, MPa (psi); Cc = clear cover, mm (in); kb = bond-dependent coefﬁcient.

The experimental bond coefficient is kb = 1.0 when FRP bars have the same bond characteristics as carbon steel, while a bond coefficient less than 1.0 indicates a superior bond between FRP bars and concrete in comparison to steel. On the other hand, a bond coefficient greater than 1.0 indicates that the FRP bond performance is inferior to traditional steel. Studies on various fiber manufacturers, cross-sections, and resin types indicate that the bond coefficient could vary from 0.6 to 1.72 [2]. ACI440.1R recommends a value of kb = 1.4 to be used in the absence of test data. The variation in the type of fiber (carbon, aramid, glass, and basalt) contributes to the reported wide range of kb values. The recommended bond coefficient for sand-coated, fiber-wrapped BFRP bars in CSA [23] is kb = 0.8, indicating the superior performance of BFRP bars compared to steel. Calculation of the bond coefficient is commonly obtained from the crack width model in CSA and is represented by Equation (3). r F 2 f s 2 w = 2 βkb + dc (3) Ef 2

where: w = maximum on the tension side; Ef = modulus of elasticity of FRP; Ff = flexural stress in FRP bars; β = ratio of the distance from the neutral axis to extreme tension fiber to distance from neutral axis to center of tensile reinforcement; dc = thickness of concrete cover measured from extreme tension fiber to center of tension bars s = spacing of longitudinal bars. Four-point loading tests on concrete beams reinforced with deformed, sand-coated, and wrapped BFRP bars indicate superior bond performance, with kb = 0.76 [11]. This study was conducted on 2.7 m clear span simply supported beams prepared using 42.5 MPa concrete compressive strength, and reinforced with 10, 12, and 16 mm diameter BFRP bars. Experimental studies by [27] deduced a bond coefficient kb = 0.77 for double sand coating of wrapped BFRP bars and kb = 0.92 for single sand coating. Clearly the bond coefficient of double sand coating is close to the recommended CSA [23] value but far less than 1.4 recommended by [2]. The calculation of the bond coefficient in this study was based on the assumed crack width of 0.7 mm, which is the upper limit of the crack width indicated for aesthetic purposes in [2]. The bond coefficient for any type of bundled BFRP bars is higher than individual bars. The reduced bond effectiveness in bundled BFRP bars, as indicated by the higher kb value, is due to the reduced area in contact with concrete. This is consistent with the Polymers 2021, 13, 1402 9 of 23

requirement for longer development length when traditional carbon reinforcing steel is bundled. kb = 1.25 was recommended for wrapped BFRP bars that are sand coated [27]. The bond coefﬁcient is often calculated based on controlled crack width, as indicated earlier. It may also be determined based on estimated service load which is sometimes taken as the load causing a stress level of 0.25 f f u ( f f u = ultimate tensile strength of BFRP) or 0.3Mn (Mn = nominal ﬂexural capacity). Studies indicate that kb calculated based on service load is lower than the values calculated based on controlled crack width [27].

3.4. Effect of Strain Rate on Bond Strength Structures may be subjected to dynamic loads, such as those caused by earthquakes or blasts, which are often simulated experimentally by applying strains at a faster rate. Studies on 10 mm diameter BFRP bars showed that bond strength increases while slip decreases with increasing strain rate. The strain rate included in the study ranged from 3.68 × 10−4 s−1 (simulating static loading) to 3.68 × 10−1 s−1 [28]. However, studies by [29] show that the bond strength of BFRP bars in concrete made with sea sand decreases with an increase in loading rate. The study covered test machine strain rates ranging from 6.4 × 10−5 s−1 (simulating static loading) to 51.3 s−1 (simulating impact). Sea sand is often used to diversify the use of natural sand resources, and it is often the least expensive alternative structure in contact with seawater.

3.5. Effect of Temperature on Bond Strength A legitimate concern on the use of FRP reinforcing bars is their performance under elevated temperatures. While basalt fibers are naturally fire resistant, the resin that binds the fibers together cannot withstand elevated temperatures. In general, the bond strength between FRP bars and concrete decreases with an increase in temperature. However, BFRP bars maintained better bond strength compared to GFRP bars at elevated temperatures ranging from 70 ◦C to 220 ◦C[30]. The loss of bond strength at 220 ◦C of BFRP bars was 7.11% compared to the bond strength of the bars at room temperature. However, at 270 ◦C, BFRP bars lost nearly 32% of their bond strength, but at 350 ◦C, the loss in bond strength was significantly higher [30].

4. Flexural Response and Contribution to Compression Forces Longitudinal GFRP and CFRP reinforcing bars used in columns contributed as little as 5% to 12% of the ultimate axial compression capacity, therefore, ACI440.1R recommends neglecting all contributions to compression in columns as well as beams [2]. Studies have shown that equivalent columns reinforced with either GFRP or BFRP have largely the same capacity and respond similarly [31], which is not surprising as the response is likely dominated by the resin and resin–fiber interface. Some studies reported the BFRP contribution to be as high as 24% of the axial ultimate compression capacity when 7% BFRP bars are used as primary longitudinal reinforcement when steel ties are used [31]. The contribution of BFRP reinforcement to the ultimate compression capacity is influenced by the amount of reinforcement, the shape of the column cross-section, concrete strength, and type of ties (material, strength, and spacing). Despite concerns over the effect of creep on strength of FRP bars in general, the contribution of BFRP bars to compression capacity deserves further investigations and need not be completely discounted. In the sequent subsections of this article, the discussion is limited to the response of flexural members reinforced with BFRP bars placed on the tension side as is typically the case in beams. The discussion in this article generally applies to the flexural response of two-way floor slabs reinforced with FRP bars. However, some experimental studies and reviews also showed FRP bars with a promising ability to resist two-way shear (punching shear) in concrete flat slab/plate, although further research is needed to understand failure modes and quantify limitations [32]. Polymers 2021, 13, 1402 10 of 23

Polymers 2021, 13, x FOR PEER REVIEW 10 of 24

4.1. Ultimate Load and Cracking Pattern FlexuralFlexural members members reinforced reinforced with with BFRP BFRP bars ba exhibitrs exhibit many many similarities similarities to the responseto the re- ofsponse beams of reinforced beams reinforced with carbon with steel. carbon As steel. indicated As indicated earlier in earlier this article, in this thearticle, modulus the mod- of elasticityulus of elasticity of BFRP barsof BFRP (44 to bars 72 MPa) (44 to is much72 MP lowera) is much than thelower typical than modulus the typical of carbon modulus steel of (200carbon MPa), steel which (200 MPa), influences which flexural influences response. flexur Load–deflectional response. Load–deflection tests of BFRP-reinforced tests of BFRP- beamsreinforced showed beams that showed stiffness that remains stiffness essentially remains the essentially same until the cracking same until begins, cracking regardless begins, ofregardless reinforcement of reinforcement ratio, and whether ratio, and or whethe not ther beamsor not the have beams shear have reinforcement shear reinforcement [16]. As shown[16]. As in shown Figure in6, Figure the load–deflection 6, the load–deflection relationship relationship of BFRP-reinforced of BFRP-reinforced beams remainsbeams re- largelymains linearlargely until linear cracking, until cracking, whether whether the beams the arebeams under-reinforced, are under-reinforced, balanced, balanced, or over- or reinforced.over-reinforced. After After cracking, cracking, the stiffness the stiffness decreases decreases as expected, as expected, but thebut load–deflectionthe load–deflec- relationshiption relationship remains remains linear linear until failure,until failu whichre, which may occur may dueoccur to due stirrup to stirrup rupture rupture when BFRPwhen stirrups BFRP stirrups are used, are as used, shown as inshown Figure in6 Fi[ gure16]. Over-reinforced6 [16]. Over-reinforced BFRP beams BFRP arebeams stiffer are thanstiffer tension-controlled than tension-controlled and balanced and balanced BFRP concrete BFRP concrete beams. beams. Therefore, Therefore, they experience they expe- slightlyrience slightly less deflection less deflection under under the same the servicesame service load, load, compared compared to balanced to balanced and and under- un- reinforcedder-reinforced beams. beams. For example, For example, at a deflection at a deflec limittion of limit L/180, of it L/180, is clear it thatis clear the compression-that the com- controlledpression-controlled beam (over-reinforced) beam (over-reinforced) carries more carries load compared more load to compared the balanced to andthe balanced tension- controlledand tension-controlled beams. beams.

120

100 L / 360 L / 180 80

60 Tension Controlled Load (kN) 40 Balanced Reinforcement 20 Compression Controlled 0 0 1020304050607080 Midspan Deflection (mm)

FigureFigure 6. 6. Load–deﬂectionLoad–deflection relationship for for beams beams reinforced reinforced with with BFRP BFRP bars bars and and BFRP BFRP stirrups stirrups Reprinted with permission from ref. [16]. Copyright 2021 American Society of Civil Engineers. Reprinted with permission from ref. [16]. Copyright 2021 American Society of Civil Engineers.

ForFor beams beams loaded loaded to failureto failure in ﬂexure,in flexure, the crackthe crack width width and distribution and distribution were enhanced were en- byhanced increasing by increasing the reinforcement the reinforcement ratio. This ratio. enhanced This responseenhanced was response attributed was toattributed increased to stiffnessincreased with stiffness increased with BFRP increased reinforcement. BFRP reinforcement. Crack width, Crack however, width, is however, not a durability is not a concerndurability in BFRP-reinforcedconcern in BFRP-reinforced beams as thebeams bars as do the not bars corrode, do not unlikecorrode, steel-reinforced unlike steel-re- beams.inforced Crack beams. width Crack is only width an aestheticis only an concern aesthetic rather concern than rather a safety than problem. a safety Therefore, problem. forTherefore, aesthetic for considerations, aesthetic considerations, a larger crack a larger width crack of 0.5 width mm of is 0.5 permitted mm is permitted for exterior for exposureexterior exposure while 0.7 while mm is 0.7 permitted mm is pe forrmitted interior for exposure interior exposure [23]. [23]. ExperimentalExperimental studiesstudies byby [[15]15] showshow that th thee ultimate moment moment capacity capacity of of beams beams rein- re- inforcedforced with with BFRP BFRP bars bars is higher thanthan thethe capacitycapacity ofof similarsimilar steel-reinforced steel-reinforced concrete concrete beamsbeams having having the the same same reinforcement reinforcement ratio. ratio. However, However, this isthis not is surprising not surprising given thegiven much the highermuch higher ultimate ultimate tensile tensile strength strength of BFRP of BFRP reinforcing reinforcing bars. bars. However, However, slabs slabs reinforced reinforced by conventionalby conventional steel steel bars bars exhibited exhibited a higher a higher moment moment than than similar similar slabs slabs reinforced reinforced with with the samethe same BFRP BFRP reinforcement reinforcement ratio. ratio. This mayThis bema attributedy be attributed to the to development the development of membrane of mem- actionbrane in action slabs reinforcedin slabs reinforced with steel with as the steel steel as is the able steel to yield, is able while to yield, BFRP willwhile continue BFRP will to carrycontinue the load to carry until the failure, load withoutuntil failure, yielding. without yielding. TheThe much much higher higher stiffness stiffness of of steel steel reinforcing reinforcing bars bars compared compared to to BFRP BFRP bars bars increases increases thethe overall overall beam beam stiffness. stiffness. The The experimental experimental study study by by Shamass Shamass and and Cashell Cashell on on the the beam beam shownshown in in Figure Figure7 7conﬁrms confirms the the reduction reduction in in beam beam stiffness stiffness when when reinforced reinforced with with BFRP BFRP bars, without decreasing the load-carrying capacity of an equivalent beam reinforced with

Polymers 2021, 13, 1402 11 of 23

Polymers 2021, 13, x FOR PEER REVIEW 11 of 24

bars, without decreasing the load-carrying capacity of an equivalent beam reinforced with carboncarbon steel. steel. FigureFigure7 7shows shows that that a a steel-reinforced steel-reinforced beam beam (S-B10-1) (S-B10-1) was was much much stiffer stiffer than than thethe BFRP BFRP beams beams (SA-B10-2, (SA-B10-2, SA-B10-1, SA-B10-1, R-B10-2, R-B10-2, and and R-B10-1). R-B10-1).

30 S-B10-1

Load (kN) 20 R-B10-2 R-B10-1 10 SA-B10-2 SA-B10-1 0 0 1020304050 Midspan Deflection (mm)

FigureFigure 7. 7.Steel-reinforced Steel-reinforced beam beam (S-B10-1) (S-B10-1) is is much much stiffer stiffer than than similar similar BFRP-reinforced BFRP-reinforced beams. beams.

InIn general, general, BFRP-reinforced BFRP-reinforced flexural flexural members members exhibit exhibit significant significant cracking cracking and and large large deflections,deflections, butbut theythey developdevelop fastfast withwith aa limited limited warning warning before before failure failure [ 33[33].]. The The large large deflectionsdeflections of of flexural flexural members members are are caused caused by by the the BFRP BFRP bars’ bars’ ability ability to to undergo undergo large large elastic elas- tensiletic tensile deformations. deformations. However, However, the ductility the duct commonlyility commonly experienced experienced in under-reinforced in under-rein- steel-reinforcedforced steel-reinforced beams beams is not seenis not in seen traditional in traditional BFRP-reinforced BFRP-reinforced flexural flexural members members [2]. As[2]. BFRP As BFRP bars bars will failwill by fail sudden by sudden tensile tensile rupture, rupture, there there is no is clear no clear preference preference for tension- for ten- controlledsion-controlled concrete concrete sections, sections, compared compared to compression-controlled to compression-controlled sections reinforced sections withrein- BFRPforced bars. with SomeBFRP bars. researchers Some researchers see a marginal see a advantagemarginal advantage in designing in designing FRP sections FRP sec- as compressiontions as compression controlled controlled by concrete by crushingconcrete thancrushing FRP than tensile FRP rupture, tensile due rupture, to the due inelastic to the deformationsinelastic deformations associated associated with concrete withcrushing concrete [crushing34]. However, [34]. However, ultimate flexural ultimate capacity flexural increasescapacity withincreases an increase with an in increase flexural reinforcement,in flexural reinforcement, regardless of regardless the failure of mode. the failure In all cases,mode. whether In all cases, the section whether is designedthe section as tensionis designed controlled as tension or compression controlled or controlled, compression the strengthcontrolled, and the serviceability strength and requirements serviceability of requirements the design must of the be met.design must be met. 4.2. Cracking Moment and Load 4.2. Cracking Moment and Load The cracking moment is a property of the concrete section, which depends on the The cracking moment is a property of the concrete section, which depends on the modulus of rupture and cross-sectional dimensions. It represents the moment correspond- modulus of rupture and cross-sectional dimensions. It represents the moment corre- ing to the initiation of flexural rupture and inception of the first crack. A fundamental sponding to the initiation of flexural rupture and inception of the first crack. A fundamen- assumption in its calculation is that it does not depend on the type of reinforcement, as tal assumption in its calculation is that it does not depend on the type of reinforcement, shown in Equations (4)–(7) for the American Concrete Institute (ACI) code, Canadian code, as shown in Equations (4)–(7) for the American Concrete Institute (ACI) code, Canadian Russian Code, and European code (EC), as well as the mechanics expression for cracking code, Russian Code, and European code (EC), as well as the mechanics expression for moment in Equation (8). cracking moment in Equation (8). p 0 fr,ACI = 0.62 fc (4) 𝑓, =0.62p 0 𝑓 (4) fr,CAN = 0.6 fc (5) 2 150 3 (5) fr,RUS𝑓,= 0.23=0.6fcu 𝑓 (6)

2 0 3 fr EC = 0.3 f (7)(6) 𝑓,, 2 =0.23 c(𝑓 ) fr Ig Mcr = 2 (8) yt ′ 3 (7) 𝑓𝑟,𝐸𝐶2 =0.3 𝑓𝑐

Polymers 2021, 13, 1402 12 of 23

The cracking moment may also be estimated experimentally by noting the cracking load, which corresponds to the change of stiffness of the experimentally developed load- deflection curve. An experimental study by [15] indicates that Equation (8) overestimates the cracking moment for BFRP-reinforced rectangular beams, especially when the bars are sand treated, which is consistent with the finding of [35]. The same equation slightly underestimates the cracking moment of an equivalent beam reinforced with traditional carbon steel. This phenomenon, tested by Shamass and Cashell, is consistent amongst codes, especially ACI318 [36] and the Canadian code [23]. In experimental research by [11], the experimental cracking moment of BFRP-reinforced beams was 24% to 27% lower than the values predicted using Equation (8). However, the BFRP reinforcement ratio did not correlate or affect the cracking moment. It is worth noting that BFRP beams are generally designed as over-reinforced, with a ratio reinforcement ratio larger than the balanced ratio. Similarly, experimental data by [27] shows that the experimentally measured cracking load Pcr for BFRP-reinforced beams was higher than that predicted values using ACI440.1R-15 or CSA, while the measured cracking load was very close to code-based predicted values for beams reinforced with traditional carbon steel. As indicated in the previous section, studies consistently showed that cracking mo- ments exhibited by BFRP-reinforced flexural members are higher than those predicted by various codes, although by different amounts. Similarly, the modulus of rupture increases by adding an adequate amount of basalt fibers, such as basalt macrofibers (BMFs), to the concrete mix [37]. More than 0.5% and up to 2% BMFs by concrete volume were shown to increase the modulus of rupture compared to slabs that were reinforced with BFRP without BMFs. In general, the effect of basalt fibers on fresh and mechanical properties of concrete depends on the mechanical and geometric properties of the basalt fibers themselves, in addition to concrete mix constituents and additives. Studies have shown that in the presence of fly ash, a dosage of 1% basalt fibers by volume produced optimum results for concrete strength and resistance to chloride penetration resistance, compared to higher or lower dosages [38].

4.3. Fatigue and Creep of BFRP Reinforcing Bars FRP bars in general, including BFRP, have low compression capacity, therefore, their use is mostly investigated to reinforce ﬂexural members including beams and slabs. Appli- cations in ﬂexural members of bridge structures are likely to require a proper understanding of fatigue response. A study by [30] on BFRP-reinforced sea sand concrete beams proposed a threshold load level of 0.55fu. ACI440.1R-15 [2] sets a much more stringent fatigue stress limit of 0.2fu for structural elements reinforced with GFRP bars, and 0.3fu for AFRP bars. However, a limit of 0.55fu is set by ACI441.1R-15 for CFRP bars, which is similar to the recommendation of given by [30] for BFRP bars. Fatigue stress is proportional to the service sustained and fatigue loads and is calculated using Equation (9).

n f d(1 − k) f f s,sus = Ms,sus (9) Icr where

Ms,sus: the maximum service (unfactored) moment due to all sustained and fatigue loads combined; k: the ratio of neutral axis depth to effective depth; Icr: the cracked moment of inertia.

bd3 I = k3 + n A d2 (1 − k)2 (10) cr 3 f f r 2 k = 2 ρ f n f + ρ f n f − ρ f n f (11) Polymers 2021, 13, 1402 13 of 23

Additionally, the reinforcement ratio of FRP bars is deﬁned as

A f ρ = (12) f bd Sustained loads over an extended period of time cause progressive deformation in FRP bars, known as creep. Flexural members subjected to bending due to various external loads, including sustained loads, will induce tensile stresses in reinforcing FRP bars. After a period of time, known as endurance time, sustained tensile stresses and resulting creep can cause a failure in FRP bars known as creep rupture. When stresses are sustained, a reinforcing BFRP bar will fail at values much lower their ultimate strength of BFRP bars. It is therefore important to control the tensile stress level in FRP bars. Creep of BFRP bars is complex and affected by the properties of ﬁbers, resin, and the interface between ﬁber and resin, and it is often assumed with reasonable accuracy that the creep of FRP bars is dominated by the resin properties [39]. It is for the most part the susceptibility of the resin to creep that makes FRP bars have a lower creep–rupture threshold compared to traditional steel bars. The most suitable mathematical model relating the ratio of the creep stress at failure/the initial ultimate strength to the sustained load duration (time) is found to be a linear relationship with logarithmic time [40–42].

4.4. Nominal Flexural Capacity When FRP reinforcing bars rupture, failure of the reinforced flexural member is catastrophic, regardless of the fiber used (carbon, glass, aramid, or basalt). Compared to steel-reinforced flexural members, FRP-reinforced beams offer a limited warning of impending failure in the form of extensive cracking and large deflections. These large deflections of the concrete membrane at failure are caused by the large elastic elongation of FRP bars before their rupture. This is especially true when the section is designed as tension controlled where failure is controlled rupture of FRP bars. In addition, the much smaller modulus of elasticity of the FRP reinforcing bars also contributes to deflections that are much larger than an equivalent steel-reinforced flexural member. The addition of 43 mm long basalt microfibers (BMFs) having 1000 MPa tensile strength tends to increase the stiffness of concrete slabs reinforced with BFRP bars when the volume fraction of BMFs exceeds 0.5%. This is demonstrated by a decrease in deflection of over-reinforced (ρ f = 1.4 ρ f b and ρ f = 2.8 ρ f b) BFRP-reinforced slabs as the volume fraction of BMFs increases from 0.5% to 2.00% [37]. ACI440.1R-15 uses a design philosophy for concrete flexural members reinforced with BFRP similar to the philosophy adopted by ACI318 [43], which recognizes the differences between FRP bars and traditional carbon steel bars, such as the fact that FRP bars do not yield but fail by tension rupture. At ultimate conditions, whether the flexural member fails by concrete crushing or FRP rupture, concrete reaches the ultimate compressive strength of εcu = 0.003, as shown in Figure8. Failure of the flexural member is controlled by tension rupture of FRP bars when the reinforcement ratio ρ f is less than the balanced reinforcement ratio ρ f b. When tension rupture controls, as shown in Figure8c, the nominal flexural capacity, Mn, for a section with an area of FRP bars, A f , is given by Equation (13).

β c M = A f d − 1 (13) n f f u 2 Polymers 2021, 13, x FOR PEER REVIEW 14 of 24

∗ 𝑓 = guaranteed tensile strength of FRP bars, defined as the mean tensile strength of a ∗ sample of test specimen minus three times standard deviation (𝑓 = 𝑓, −3𝜎), MPa (psi). The environmental reduction factor, 𝐶, is taken as 0.8 or 0.7 [2] depending on the Polymers 2021, 13, 1402severity of exposure conditions for FRP bars made of glass fibers, while the factor is 0.9 or 14 of 23 1.0 for carbon fibers. No data are provided for BFRP bars, but current research is covered in a subsequent section of this article.

Figure 8. Strain andFigure stress 8. distributionStrain and stress at ultimate distribution conditions at ultimate [2]. conditions [2].

5. Shear Strength andThe Response design tensile of BFRP-Reinforced strength f f u, given Beams by and Equation Transverse (14), Shear considers of the long-term Bars effects of the environment (such as marine and alkaline environments) on degrading the Unlike conventionaltensile strength. reinforcing steel bars, FRP bars are anisotropic, characterized by = ∗ high tensile strength in the direction of fibers. This affectsf f u CtheE fshearf u strength and dowel (14) action of FRP barswhere: [2]. Transverse shear resistance of FRP bars is relatively low and largely dominated by the polymer, which negatively impacts its contribution to the shear re- f ∗ = guaranteed tensile strength of FRP bars, defined as the mean tensile strength of a sistance of the entiref u concrete reinforced with FRP bars. A test procedure for estimating f ∗ = f − σ) the transverse shearsample of ofthe test FRP specimen bars, in general, minus threeis described times standard by [44]. deviation ( f u f u, average 3 , Most designMPa codes (psi). and standards consider the shear capacity of a concrete cross-section, Vn, to be the combinationThe environmental of shear reductionresistance factor,providedCE, by is takenconcrete as 0.8mechanisms, or 0.7 [2] depending Vc, on the and shear resistanceseverity provided of exposure by the conditions reinforcing for FRP FRP stirrups, bars made Vf. Studies of glass have fibers, shown while that the factor is 0.9 or the shear capacity1.0 forof concrete, carbon fibers. Vc, is Noinfluenced data are by provided the axial for stiffness BFRP bars, of tension but current reinforce- research is covered ment (product inof a the subsequent modulus section of elasticity of this times article. the tension reinforcement area) [45]. Other studies on beams reinforced by BFRP bars also indicate that with or without BFRP 5. Shear Strength and Response of BFRP-Reinforced Beams and Transverse Shear of Bars Unlike conventional reinforcing steel bars, FRP bars are anisotropic, characterized

by high tensile strength in the direction of ﬁbers. This affects the shear strength and dowel action of FRP bars [2]. Transverse shear resistance of FRP bars is relatively low and largely dominated by the polymer, which negatively impacts its contribution to the shear Polymers 2021, 13, 1402 15 of 23

resistance of the entire concrete reinforced with FRP bars. A test procedure for estimating the transverse shear of the FRP bars, in general, is described by [44]. Most design codes and standards consider the shear capacity of a concrete cross- section, Vn, to be the combination of shear resistance provided by concrete mechanisms, Vc, and shear resistance provided by the reinforcing FRP stirrups, Vf. Studies have shown that the shear capacity of concrete, Vc, is influenced by the axial stiffness of tension reinforcement (product of the modulus of elasticity times the tension reinforcement area) [45]. Other studies on beams reinforced by BFRP bars also indicate that with or without BFRP shear reinforcement, the load at which the first diagonal shear crack occurs increases with the flexural reinforcement ratio [16]. Despite the high ultimate tensile strength fu, the axial stiffness of FRP bars is lower than steel bars of the same area, due to the lower modulus of elasticity of FRP bars in general. As a result of the relatively lower axial stiffness, the neutral axis depth of the cracked FRP-reinforced concrete section is shallower than an equivalent concrete section with an equal area of reinforcing steel. As a result, ACI440.1R- 15 recommends the concrete contribution to shear resistance given by Equation (15), which depends on the neutral axis depth kd. The neutral axis depth kd is dependent on the ratio of FRP reinforcement ratio, ρ f , and the modular ratio, n f .

2 p V = f 0 b (kd) (15) c 5 c w r 2 k = 2 ρ f n f + ρ f n f − ρ f n f (16)

where

ρ f = fiber-reinforced polymer reinforcement ratio; n f = ratio of modulus of elasticity of FRP bars to the modulus of elasticity of concrete; Vf = shear resistance provided by FRP stirrups, N (lb); Vn = nominal shear strength at section, N (lb); Vs = shear resistance provided by steel stirrups, N (lb); Vu = factored shear force at section, N (lb); w = maximum allowable crack width, mm (in); f f v = tensile strength of FRP for shear design, taken as smallest of design tensile strength; f f u = the strength of the bent portion of FRP stirrups f f b, or stress corresponding to 0.004Ef , MPa (psi); ρ f v = ratio of FRP shear reinforcement. Noting the effect of the relatively low axial stiffness of FRP bars in general compared to steel reinforced concrete members on shear strength, it is therefore beneficial to consider increasing the reinforcement ratio and design concrete flexural members as over-reinforced. Increasing the reinforced ratio and/or modulus of elasticity of BFRP was shown to reduce shear crack width and increase the contribution of uncracked concrete to shear resistance by increasing the depth of the compression block and aggregate interlock [46]. BFRP shear reinforcement placed perpendicular to the axis of the member is effective in resisting shear failure and increasing the load-carrying capacity, especially when the beam is tension controlled [16]. BFRP shear reinforcing stirrups also increase resistance to shear failure in compression-controlled beams, but to a lesser extent compared to tension- controlled beams. As shear rupture of the stirrups is common when beams are reinforced with BFRP stirrups, ACI440.1R-15 places a strict limit on the stress in stirrups, f f v, as given by Equation (17). The strict stress limit also controls crack widths, which are wider in FRP-reinforced beams, and ensures the integrity of the beam by avoiding failure at the bent portion of the FRP stirrups. Such a limit, however, needs investigation for BFRP stirrups as ACI4410.1R-15 does not explicitly address reinforcing bars made of basalt fibers.

f f v = 0.004 Ef ≤ f f b (17) Polymers 2021, 13, 1402 16 of 23

where ffb = strength of the bent portion of FRP, MPa (psi). The sheer force carried by the stirrups is proportional to the stress, f f v, in FRP stirrups, longitudinal spacing of FRP stirrups along the beam, s, and area of the vertical legs of stirrups, A f v. For CFRP, AFRP, and GFRP, ACI4401.R-15 adopts the mechanics-based Equation (18), which will likely remain the same for BFRP stirrups when incorporated in the standard. A f v f f vd V = (18) f s In seismic areas, where the use of lightweight concrete is of interest, studies have shown that BFRP-reinforced beams exhibited the same concrete contribution to shear resistance as that of normal-weight concrete [46]. That contribution to shear resistance was also the same for lightweight concrete and normal-weight concrete, whether BFRP bars were sand coated without helical wrapping, or helically wrapped without sand coating.

6. Durability It is important to develop a reasonable understanding of the long-term structural response of concrete members reinforced with BFRP bars in an unfavorable environment. The unique structure of FRP bars, consisting of ﬁbers and resin, makes it important to evaluate their durability in aggressive environments such as prolonged exposure to elevated temperature, highly alkaline environments, and freezing/thawing and low temperature. In addition, moisture ingress into the resin, before placement in concrete, could lead to degradation of mechanical properties of FRP reinforcing bars. Some studies indicate that vinyl ester resin offers better resistance to moisture ingress compared to other types of resins used in making FRP bars. Studies have shown that up to 40% of the tensile strength of GFRP bars can be lost after exposure to a combination of ultraviolet rays and moisture tests with and without loading [47]. Tensile tests of GFRP, BFRP, and CFRP bars, made of vinyl ester resin, conducted after immersion in tap water (pH = 7.00) for 180 days, showed various degrees of degradation [48]. BFRP and CFRP bars retained nearly 89% of the tensile strength, while GFRP retained 78%. Other studies conﬁrmed the superior performance of vinyl ester resin in terms of moisture uptake, where BFRP bars made with vinyl ester resin exhibited lower moisture uptake (40%) compared to BFRP bars made with epoxy resin (68%) [49]. The moisture uptake measurements were done after conditioning the BFRP bars in an alkaline solution for 5000 h at a temperature of 60 ◦C.

6.1. Properties of BFRP Bars in Relatively Elevated Temperature The bond strength between ribbed BFRP bars (without wrapping or sand coating) and concrete appears to improve in samples subjected to higher temperatures of 50 ◦C and 60 ◦C for 1.5 months, compared to samples tested after 1.5 months of exposure to 40 ◦C[50]. The reasons for such an increase are not clear, but researchers hypothesized that an increase in concrete strength at a higher temperature may be the cause of the relatively enhanced bond strength. However, bond strength decreased by 16% for samples immersed in alkaline solution for 6 months at 40 ◦C. The reduction in bond strength for similar samples subjected to higher temperatures of 50 ◦C and ◦C was lower (7% and 5%, respectively), conﬁrming the positive effect of relatively elevated temperature on bond strength.

6.2. Effect of Alkaline Environment on Properties of BFRP Bars Alkalinity is deﬁned as the condition of having or containing hydroxyl ions (OH−1)[2]. In experimental studies, alkaline solutions may be prepared using calcium hydroxide, potassium hydroxide, sodium hydroxide, etc. [51]. Aqueous solutions with pH ranging from 11.5 to 13 are known to degrade the tensile strength and stiffness of GFRP reinforcing bars [52]. It is necessary to examine the research ﬁndings on the effect of such an aggressive environment on the properties of BFRP bars. Polymers 2021, 13, 1402 17 of 23

Engineering properties of BFRP bars themselves are adversely impacted by the alkaline environment, especially at temperatures higher than 40 ◦C. Alkaline environments with pH values between 8 and 10 are likely for mortar and concrete during service life [53]. A pH of up to 13 may occur in an aggressive alkaline environment. The deterioration in properties is mostly due to the disintegration of the matrix, which accelerates at relatively high temperatures. Some of the properties that deteriorate after exposure to an alkaline environment include transverse shear strength, interlaminar shear strength, and flexural strength. The method in [54] is one of the tests to evaluate the resistance of FRP bars to deterioration of properties caused by an alkaline environment. ASTM D7705 recognizes the effect of moderately high temperature in accelerating the matrix deterioration in an alkaline environment by specifying a test temperature of 60 ◦C. It was noted that shear strength and interlaminar shear strength of larger bar diameters are less impacted by alkaline environments than smaller diameters. The higher strength retention in larger diameter BFRP bars is attributed to the smaller affected thickness [20]. On the other hand, the tensile strength retention of smaller BFRP bar diameters after exposure to an alkaline environment was higher than larger bar diameters. Al Rifai et al. [55] showed that BFRP bars lost 29% of their original tensile strength after 9 months of conditioning in an alkaline environment at 60 ◦C. Elgabbas et al. reported that after 3 months of conditioning in alkaline solution, BFRP bars lost 21.2% of the ultimate tensile strength [10]. That study found that the same BFRP bars, with 77.4% fiber content by weight, lost approximately 5% of the modulus of elasticity. The stability of the modulus of elasticity was also observed in 6 mm diameter BFRP bars made with epoxy resin and conditioned for 63 days in alkaline solution, saline solution, or even acid solution [56]. Examination under a scanning electron microscope (SEM) showed that the degradation of mechanical properties of these BFRP bars made with vinyl ester resin occurred at the fiber–matrix interface rather than in the matrix or fiber [10]. It is therefore clear that BFRP bars’ loss in the modulus of elasticity is limited after exposure to various aggressive environments. Conditioning in alkaline solution (pH = 12.8) at a relatively high temperature of 60 ◦C for 5000 h (7 months) caused a loss of 8.3% in the modulus of elasticity of BFRP bars [51]. The fiber content of 20 mm diameter BFRP bars used in this study was 81%, impregnated with vinyl ester resin through pultrusion. Studies by [57] showed that CFRP bars offered the highest tensile strength retention after conditioning in an alkaline environment for three months at 60 ◦C, followed by BFRP bars, then GFRP bars. However, the authors found that the alkali resistance of different FRP bars is sensitive to the type of resin, fiber, and manufacturing method. For instance, GFRP with vinyl ester resin was found to exceed the 80% minimum retention of tensile strength and inter-laminar shear stress after conditioning in an alkaline environment. GFRP bars made with E-glass fibers, in particular, exhibited the highest inter-laminar shear strength retention after exposure to an alkaline environment, compared to BFRP and CFRP. Alkaline resistance of BFRP bars made of polyurethane and epoxy resins is better than alkaline resistance when vinyl ester resin is used [57]. Prediction models for ultimate tensile strength and moduli of BFRP bars in an alkaline concrete/mortar environment estimate that after 100 years of exposure at temperatures up to 60 ◦C, 72% of the ultimate tensile strength and 80% of the modulus of elasticity are retained [53]. The BFRP bars were ribbed and sand coated (0.5 mm coating thickness), manufactured using epoxy resin, and included diameters from 3 mm to 10 mm. Room temperature (20 ◦C ± 2 ◦C) studies on 8 mm diameter bars made of vinyl ester matrix that were immersed in alkaline solution (pH = 12.9) for 180 days show the relative superiority of BFRP bars compared to GFRP in terms of retention of tensile strength. BFRP bars retained 77.6% of the unconditioned tensile strength while GFRP bars retained 69.2%. CFRP experienced the least deterioration in the same alkaline environment by retaining 82.8% of the unconditioned tensile strength [48], the same percentage of tensile strength after immersion in seawater solution for 180 days at room temperature. Figure9 shows the tensile strength retention percentages of 8-mm bars after immersion in alkaline solution Polymers 2021, 13, 1402 18 of 23

(pH = 12.9) seawater and tap water. GFRP bars in vinyl ester matrix are particularly Polymers 2021, 13, x FOR PEER REVIEWvulnerable to moisture compared to BFRP and CFRP as immersion in tap water for 180 days18 of 24 leads to the lowest retention of tensile strength.

45 days 90 days 135 days 180 days 120 91.9 96.7 92.9 100 88.4 83 84.3 88.9 88.5 77.6 79.1 76.8 75.8 80 60 40 (%) 20 0 Alkaline solution Seawater solution Tap water solution

Tensile Strength Retention a) Environment

45 days 90 days 135 days 180 days 120 99 96.7 94.6 92.8 91.1 92.2 90.4 100 84.6 88.9 82.8 81.4 78.8 80 60

(%) 40 20 0 Alkaline solution Seawater solution Tap water solution Tensile Strength Retention b) Emvironment

45 days 90 days 135 days 180 days 120 96.5 99.5 97.3 100 92.1 90.5 89.7 81.8 85.3 75.3 78 80 70.6 69.2 60

(%) 40 20 0 Alkaline solution Seawater solution Tap water solution Tensile Strength Retention c) Environment

FigureFigure 9. 9.Tensile Tensile strength strength retention retention in in 8 mm8 mm diameter diameter FRP FRP bars bars made made of of vinyl vinyl ester ester matrix matrix subjected subjected to three environments with bars made of (a) BFRP fibers, (b) CFRP fibers, (c) GFRP fibers. to three environments with bars made of (a) BFRP fibers, (b) CFRP fibers, (c) GFRP fibers. Reprinted Reprinted with permission from ref. [48]. Copyright 2021 American Society of Civil Engineers with permission from ref. [48]. Copyright 2021 American Society of Civil Engineers

ExperimentalExperimental studies studies conducted conducted to to date date have have produced produced data data on on bond bond durability durability with with signiﬁcantsignificant scatter scatter in in terms terms of of the the loss loss of of bond bond strength strength due due to conditioning.to conditioning. Such Such scatter scatter is causedis caused by researchby research campaigns campaigns being being designed designed with with different different goals goals set by set researchers. by researchers. As a result,As a result, bond durability bond durability models models that take that advantage take advantage of the abundant of the abundant test data test have data proved have toproved be challenging to be challenging [58]. [58].

Polymers 2021, 13, 1402 19 of 23

6.3. Response of BFRP-Reinforced Concrete Members Subjected to Freezing and Thawing Cycles and Low Temperature The performance of BFRP-reinforced concrete structures at low temperatures and subjected to freezing and thawing cycles is also of interest to researchers and designers. Exposure to low temperature and freezing/thawing (FT) cycles did not change the bond failure mode in experimental studies [59]. Up to a freezing temperature of −20 ◦C, the failure mode in the bond test was dominated by shear between BFRP bars and concrete. However, the bond strength at −20 ◦C decreased by 10% compared to original samples tested under normal temperature, without exposure to FT cycles. Studies on the effects of FT cycles on bond strength were conﬂicting. Some studies indicate that FT cycles of 100 and 200 (two cycles per day) had a limited effect on the bond strength [59], while other studies indicate a reduction in bond strength after samples were subjected to FT cycles (10 cycles) [60]. The effect of FT cycles on bond strength needs further investigation.

7. Conclusions This article reviewed the current state-of-knowledge on research related to the application of basalt fiber-reinforced polymer (BFRP) as reinforcing bars for concrete. The review emphasized mechanical properties and durability as being the most critical for engineers in professional practice as well as researchers. The most commonly researched and used fibers for FRP bars are glass, carbon, and aramid, therefore, FRP bars with these materials have been incorporated in international design guides and standards on concrete reinforced with FRP bars, such as ACI440.R-15. The use of basalt fibers to produce concrete reinforcing FRP bars is gaining popularity due to their competitive durability, natural corrosion resistance, nonmagnetic properties, and sufficiently high tensile strength. Although carbon steel retains its environmental advantage of being 100% recyclable, its manufacturing process continues to emit significant CO2 into the atmosphere. The major findings of this review include: • Similar to all FRP reinforcing bars, the BFRP stress–strain relationship is linear until failure by tensile rupture, unlike traditional reinforcing steel which reaches yield stress, becomes inelastic, strain hardens to ultimate strength, and then softens to rupture. • Characteristics of the bond between BFRP bars and the surrounding concrete are influenced by factors including rib height and rib spacing. Many studies confirm that the most effective bond strength is achieved when BFRP reinforcing bars are ribbed, helically wrapped, and sand coated. One study reported that sand-coated ribbed bars achieved better bond strength than helically wrapped and sand-coated bars. The bond coefficient of helically wrapped and sand-coated BFRP bars was found to be comparable, and sometimes superior, to the bond coefficient of traditional carbon reinforcing steel. The bond coefficient is used in design codes to control crack width, an essential serviceability consideration of FRP-reinforced flexural concrete members. Loss of bond strength of BFRP bars at elevated temperatures is equivalent to that of GFRP bars. However, at an elevated temperature of 350 ◦C, a significant loss of bonding occurs in BFRP bars. • The modulus of elasticity of FRP bars is much lower than traditional steel bars, leading to higher deflections in equivalent FRP-reinforced flexural members. As a result, the over-reinforced design offers the relative advantage of reduced deflections compared to under-reinforced FRP flexural members. Nonetheless, most standards, including ACI4401.R-15, provide guidance on designing FRP-reinforced beams as under-reinforced or over-reinforced. • Experimental studies confirmed that tensile strength, shear strength, and interlaminar shear strength deteriorate over time when BFRP bars are conditioned in alkaline solution, especially at high temperatures. At a relatively high temperature of 60 ◦C, BFRP bars were found to lose nearly 29% of the tensile strength after 9 months of conditioning in alkaline solution. Due to the larger exposure area, larger bar Polymers 2021, 13, 1402 20 of 23

diameters were more impacted by the alkaline environment than smaller diameters. Deterioration in the modulus of elasticity was much smaller than in tensile strength. • There is a need for standardization of the manufacturing of BFRP bars with the goal of providing reliable guaranteed mechanical properties, such as tensile strength and shear strength. Significant variability is reported in mechanical properties, often including bars produced by the same manufacturer. One reason for the variability in properties is the inconsistency in the application method and composition of fiber size, a thin layer of treatment applied to the fibers during manufacturing. Currently, pultrusion is the most common method for manufacturing BFRP bars. • Future research needs: Limited research is available on the structural response of cast-in-place (CIP) BFRP-reinforced concrete members under compression and/or combined compression and flexure, which currently restricts the application of BFRP reinforcement to flexural members. Further research is needed to quantify the effect of rib depth and rib spacing of deformed BFRP bars on the structural response of BFRP-reinforced flexural members. Sand coating to enhance bonding of BFRP to the surrounding concrete is still an open research area in terms of effectiveness and clarity of specifications of the materials and methods of application.

Author Contributions: Conceptualization, O.A.M.; methodology, O.A.M.; software, W.A.H.; valida- tion, W.A.H. and O.A.M.; investigation, O.A.M.; resources, W.A.H. and O.A.M.; writing—original draft preparation, O.A.M.; writing—review and editing, W.A.H., M.K., and O.A.M.; visualization, W.A.H.; funding acquisition, O.A.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Office of Research and Sponsored Programs (ORSP) at Abu Dhabi University, grant number 19300460. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

Variable Description BFRP Basalt Fiber-Reinforced Polymer kb Bond Coefﬁcient FRP Fiber-Reinforced Polymer GWP Global Warming Potential MRI Magnetic Resonance Imaging GFRP Glass Fiber-Reinforced Polymer CFRP Carbon Fiber-Reinforced Polymer AFRP Aramid Fiber-Reinforced Polymer HCC Hollow Concrete Column fu Ultimate Stress fv Allowable Stress εcu Ultimate Strain rs Rib Spacing Polymers 2021, 13, 1402 21 of 23

rh Relative Rib Area wc Concrete Lug Width wf FRP Bar Lug Width τ Average Bond Stress F Tensile Force Cb Effective Circumference of FRP Bar l Bonded Length Smax Maximum Spacing Between Tension Reinforcing FRP Bars. Design or Guaranteed Modulus of Elasticity of FRP Deﬁned as Mean Modulus Ef of Sample of Test Specimen Ef = Eaverage )

f f s Stress Level Induced in FRP at Service Load Cc Clear Cover w Maximum Crack Width on the Tension Side Ef Modulus of Elasticity of FRP Ff Flexural Stress in FRP Bars Ratio of the Distance from the Neutral Axis to Extreme Tension Fiber to β Distance from Neutral Axis to Center of Tensile Reinforcement Thickness of Concrete Cover Measured from Extreme Tension Fiber to Center of d c Tension Bars s Spacing of Longudinal Bars Mn Nominal Flexural Capacity fr,ACI Fracture Stress (American Concrete Institute) fr,CAN Fracture Stress (Cadian Code) fr,RUS Fracture Stress (Rsian Code) fr,EC2 Fracture Stress (Eopean code) Mcr Cracking Moment f f s,sus Fatigue Stress due to Sustned and Fatigue Load n f Ratio of Modulus of Elasticity of FRP Bars to the Modulus of Elasticity of Concre d Effective Depth k Ratio of Neutral Axis Deh to Effective Depth Icr Cracked Moment of Inertia A f Area of FRP Bars ρ f Fiber-Reinforced PolymeReinforcement Ratio b Width f f u Design Tensi Strength ∗ f f u Guaranteed Tenle Strength CE Environmental Reduction Factor Vn Nominal Shear Strength at Section Vc Shear Resistance Provided by Concrete Mechanisms Vf Shear Resistance Provided by the Reinforcing FRP Stirrups Vs Shear Resistance Provid by Steel Stirrups Vu Factored Shear Foe at Section w Maximum Allowab Crack Width f f v Tensile Strength of FRP for Shear Design ρ f v Ratio of FRP Shear Reinforcement f f b Strength of the Bent Portion of FRP A f v Area of FRP Stirrups s Spacing between Stirrups

References 1. Inman, M.; Thorhallsson, E.R.; Azrague, K.A. Mechanical and environmental assessment and comparison of basalt ﬁbre reinforced polymer (BFRar and steel rebar in concrete beams. Energy Procedia 2017, 111, 31–40. [CrossRef] 2. ACI Committee 440. Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer (FRP) Bars, AC440.1R; American Concrete Institute: Farmington Hills, MI, USA, 2015; ISBN 978-1-942727-10-1. 3. Alajarmeh, O.; Manalo, A.; Benmokrane, B.; Karunasena, K.; Ferdous, W.; Mendis, P. Hollow concrete columns: Review of structural behavior and new designs using GFRP reinforcement. Eng. Struct. 2020, 203, 109829. [CrossRef] Polymers 2021, 13, 1402 22 of 23

4. Ferdous, W.; Manalo, A.; Al Ajarmeh, O.; Mohammed, A.A.; Salih, C.; Yu, P.; Khotbehsara, M.M.; Schubel, P. Static behaviour of glass fibre reinforced novel composite sleepers for mainline railway track. Eng. Struct. 2021, 229, 111627. [CrossRef] 5. Manalo, A.; Maranan, G.; Benmokrane, B.; Cousin, P.; Alajarmeh, O.; Ferdous, W.; Liang, R.; Hota, G. Comparative durability of GFRP composite reinforcing bars in concrete and in simulated concrete environments. Cem. Concr. Compos. 2020, 109, 103564. [CrossRef] 6. Patnaik, A. Applications of basalt fiber reinforced polymer (BFRP reinforcement for transportation infrastructure. In Developing a Research Agenda for Transportation Infrastructure; Transportation Research Board: Washington, DC, USA, 2009; pp. 1–5. 7. Thomason, J. Glass fibre sizing: A review. Compos. Part A Appl. Sci. Manuf. 2019, 127, 105619. [CrossRef] 8. Loewenstein, K.L. Glass Science and Technology 6. The Manufacturing Technology of Continuous Glass Fibers; Elsevier: Amsterdam, The Netherlands, 1993. 9. Mohamed, O.A.; KewalRamani, M.; Khattab, R. Fiber Reinforced Polymer Laminates for Strengthening of RC Slabs against Punching Shear: A Review. Polymer 2020, 12, 685. [CrossRef] 10. Elgabbas, F.; Ahmed, E.A.; Benmokrane, B. Physical and mechanical characteristics of new basalt-FRP bars for rein-forcing concrete structures. Constr. Build. Mater. 2015, 95, 623–635. [CrossRef] 11. Elgabbas, F.; Vincent, P.; Ahmed, E.A.; Benmokrane, B. Experimental testing of basalt-fiber-reinforced polymer bars in concrete beams. Compos. Part B Eng. 2016, 91, 205–218. [CrossRef] 12. Banibayat, P.; Patnaik, A. Creep Rupture Performance of Basalt Fiber-Reinforced Polymer Bars. J. Aerosp. Eng. 2015, 28, 04014074. [CrossRef] 13. Bisby, L.; Kodur, V. Evaluating the fire endurance of concrete slabs reinforced with FRP bars: Considerations for a holistic approach. Compos. Part B Eng. 2007, 38, 547–558. [CrossRef] 14. Nigro, E.; Bilotta, A.; Cefarelli, G.; Manfredi, G.; Cosenza, E. Performance under fire situations of concrete members reinforced with FRP rods: Bond Models and Design Nomograms. J. Comp. Constr. 2012, 16, 395–406. [CrossRef] 15. Shamass, R.; Cashell, K.A. Experimental investigation into the flexural behavior of basalt FRP reinforced concrete members. Eng. Struct. 2020, 220, 110950. [CrossRef] 16. Tomlinson, D.; Fam, A. Performance of Concrete Beams Reinforced with Basalt FRP for Flexure and Shear. J. Compos. Constr. 2015, 19, 04014036. [CrossRef] 17. Wang, Z.; Zhao, X.; Xian, G.; Wu, G.; Raman, S.R.; Al-Saadi, S.; Haque, A. Long-term durability of basalt- and glass-fiber reinforced polymer (BFRP/GFRP) bars in seawater and sea sand concrete environment. Constr. Build. Mater. 2017, 139, 467–489. [CrossRef] 18. Arias, J.P.M.; Vazquez, A.; Escobar, M.M. Use of sand coating to improve bonding between GFRP bars and concrete. J. Compos. Mater. 2012, 45, 2271–2278. [CrossRef] 19. Rosa, I.C.; Firmo, J.P.; Correia, J.R.; Barros, J.A.O. Bond behavior of sand-coated GFRP bars to concrete at elevated temperature— Definition of bond vs. slip relations. Comp. Part B 2019, 160, 329–340. [CrossRef] 20. Ali, A.H.; Mohamed, H.M.; Benmokrane, B. Bar size effect on long-term durability of sand-coated basalt-FRP composite bars. Comp. Part B 2020, 195, 108059. [CrossRef] 21. Gu, X.; Dong, Q. Laboratory test and numerical simulation of bond performance between basalt fiber reinforced polymer rebar and concrete. J. Test. Eval. 2012, 40, 1148–1155. [CrossRef] 22. Solyom, S.; Balázs, G.L. Bond of FRP bars with different surface characteristics. Constr. Build. Mater. 2020, 264, 119839. [CrossRef] 23. CAN/CSA-S806-12. Design and Construction of Building Structures with Fiber Reinforced Polymers; Canadian Standard Association: Ottawa, ON, Canada, 2012. 24. Zou, S.; Liu, F.; Xiong, Z.; He, S.; Li, L.; Wei, W. Experimental study on fatigue bond behaviour between basalt fi-bre-reinforced polymer bars and recycled aggregate concrete. Constr. Build. Mater. 2020, 270, 121399. [CrossRef] 25. ASTM D7913/D7913M-14. Standard Test Method for Bond Strength of Fiber-Reinforced Polymer Matrix Composite Bars to Concrete by Pull-Out Testing; ASTM International: West Conshohocken, PA, USA, 2014. 26. China Academy of Building Research. Code for Design of Concrete Structures GB50010-2010; China Architecture & Building Press: Beijing, China, 2017. 27. Henin, E.; Tawadrous, R.; Morcous, G. Effect of surface condition on the bond of Basalt Fiber-Reinforced Polymer bars in concrete. Constr. Build. Mater. 2019, 226, 449–458. [CrossRef] 28. Shen, D.; Li, C.L.; Feng, Z.; Wen, C.; Ojha, B. Influence of strain rate on bond behavior of concrete members rein-forced with basalt fiber-reinforced polymer rebar. Constr. Build. Mater. 2019, 228, 116755. [CrossRef] 29. Wei, W.; Liu, F.; Xiong, Z.; Yang, F.; Li, L.; Luo, H. Effect of loading rate on bond behavior between basalt fiber reinforced polymers bars and concrete. Constr. Build. Mater. 2020, 231, 117138. [CrossRef] 30. Li, C.; Gao, D.; Wang, Y.; Tang, J. Effect of high temperature on the bond performance between basalt fibre reinforced polymer (BFRP) bars and concrete. Constr. Build. Mater. 2017, 141, 44–51. [CrossRef] 31. AlNajmi, L.; Abed, F. Evaluation of FRP Bars under Compression and Their Performance in RC Columns. Materials 2020, 13, 4541. [CrossRef][PubMed] 32. Mohamed, O.A.; Khattab, R. Review of Punching Shear Behaviour of Flat Slabs Reinforced with FRP Bars. IOP Conf. Ser. Mater. Sci. Eng. 2017, 245, 32064. [CrossRef] Polymers 2021, 13, 1402 23 of 23

33. Jaeger, L.G.; Mufti, A.; Tadros, G. The Concept of the Overall Performance Factor in Rectangular-Section Rein-forced Concrete Beams. In Proceedings of the 3rd International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Sapporo, Japan, 14–16 October 1997; Volume 2. 34. Nanni, A. Flexural Behavior and Design of RC Members Using FRP Reinforcement. J. Struct. Eng. 1993, 119, 3344–3359. [CrossRef] 35. Duic, J.; Kenno, S.; Das, S. Performance of concrete beams reinforced with basalt fibre composite rebar. Constr. Build. Mater. 2018, 176, 470–481. [CrossRef] 36. ACI Committee 318. Building Code Requirements for Structural Concrete (ACI 318M-14) and Commentary (ACI 318RM-14); American Concrete Institute: Farmington Hills, MI, USA, 2015; ISBN 978-1-942727-11-8. 37. Attia, K.; Alnahhal, W.; Elrefai, A.; Rihan, Y. Flexural behavior of basalt fiber-reinforced concrete slab strips reinforced with BFRP and GFRP bars. Comp. Struct. 2019, 211, 1–12. [CrossRef] 38. Mohamed, O.A.; Al-Hawat, W. Influence of fly ash and basalt fibers on strength and chloride penetration resistance of self- consolidating concrete. Mater. Sci. Forum 2016, 866, 3–8. [CrossRef] 39. Karbhari, V.M.; Chin, J.W.; Hunston, D.; Benmokrane, B.; Juska, T.; Morgan, R. Durability gap analysis for fiber-reinforced polymer composites in civil infrastructure. J. Compos. Constr. 2003, 238, 238–247. [CrossRef] 40. Yamaguchi, T.; Kato, Y.; Nishimura, T.; Uomoto, T. Creep Rupture of FRP Rods Made of Aramid, Carbon and Glass Fibers. In Proceedings of the Sixth East Asia-Pacific Conference on Structural Engineering & Construction (EASEC 6), Taipei, Taiwan, 14–16 January 1998; pp. 179–186. 41. Seki, H.; Sekijima, K.; Konno, T. Test Method on Creep of Continuous Fiber Reinforcing Materials. In Proceedings of the 3rd International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Sapporo, Japan, 14–16 October 1997; pp. 195–202. 42. Banibayat, P.; Patnaik, A. Variability of mechanical properties of basalt fiber reinforced polymer bars manufactured by wet-layup method. Mater. Des. 2014, 56, 898–906. [CrossRef] 43. ACI318-19. Building Code Requirements for Structural Concrete (ACI318-19) & Commentary; American Concrete Institute—ACI318-19: Farmington Hills, MI, USA, 2019. 44. D30 Committee. Test Method for Transverse Shear Strength of Fiber-Reinforced Polymer Matrix Composite Bars; ASTM International: West Conshohocken, PA, USA, 2017. 45. Tureyen, A.K.; Frosch, R.J. Shear tests of FRP reinforced concrete beams without stirrups. ACI Struct. J. 2002, 99, 427–434. 46. Mehany, S.; Mohamed, H.M.; Benmokrane, B. Contribution of lightweight self-consolidated concrete (LWSCC) to shear strength of beams reinforced with basalt FRP bars. Eng. Struct. 2021, 231, 111758. [CrossRef] 47. Sasaki, I.; Nishizaki, I.; Sakamoto, H.; Katawaki, K.; Kawamoto, Y. Durability Evaluation of FRP Cables by Exposure Tests. In Proceedings of the 3rd International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Sapporo, Japan, 14–16 October 1997; pp. 131–137. 48. Lu, C.; Ni, M.; Chu, T.; He, L. Comparative Investigation on Tensile Performance of FRP Bars after Exposure to Water, Seawater, and Alkaline Solutions. J. Mater. Civ. Eng. 2020, 32, 04020170. [CrossRef] 49. Benmokrane, B.; Elgabbas, F.; Ahmed, E.A.; Cousin, P. Characterization and Comparative Durability Study of Glass/Vinylester, Basalt/Vinylester, and Basalt/Epoxy FRP Bars. J. Compos. Constr. 2015, 19, 04015008. [CrossRef] 50. Hassan, M.; Benmokrane, B.; ElSafty, A.; Fam, A. Bond durability of basalt-fiber-reinforced-polymer (BFRP) bars embedded in concrete in aggressive environments. Comp. Part B 2016, 106, 262–272. [CrossRef] 51. Ali, A.H.; Mohamed, H.M.; Benmokrane, B.; El Safty, A.; Chaallal, O. Durability performance and long-term pre-diction models for sand-coated basalt FRP bars. Comp. Part B 2019, 157, 248–258. [CrossRef] 52. Porter, M.L.; Barnes, B.A. Accelerated Aging Degradation of Glass Fiber Composites; Saadatmanesh, H., Eshani, M.R., Eds.; University of Arizona: Tucson, AZ, USA, 1998; pp. 446–459. 53. Serbescu, A.; Guadagnini, M.; Pilakoutas, K. Mechanical Characterization of Basalt FRP Rebars and Long-Term Strength Predictive Model. J. Compos. Constr. 2015, 19, 04014037. [CrossRef] 54. ASTM D7705/D7705M-12. Standard Method for Alkali Resistance of Fiber Reinforced Polymer (FRP) Matrix Composite Bars Used in Concrete Construction; ASTM Internationa: West Conshohocken, PA, USA, 2019. 55. Al Rifai, M.; El-Hassan, H.; El-Maaddawy, T.; Abed, F. Durability of basalt FRP reinforcing bars in alkaline solution and moist concrete environments. Constr. Build. Mater. 2020, 243, 118258. [CrossRef] 56. Wu, G.; Dong, Z.-Q.; Wang, X.; Zhu, Y.; Wu, Z.-S. Prediction of Long-Term Performance and Durability of BFRP Bars under the Combined Effect of Sustained Load and Corrosive Solutions. J. Compos. Constr. 2015, 19, 04014058. [CrossRef] 57. Benmokrane, B.; Hassan, M.; Robert, M.; Vijay, P.V.; Manalo, A. Effect of Different Constituent Fiber, Resin, and Sizing Combina- tions on Alkaline Resistance of Basalt, Carbon, and Glass FRP Bars. J. Compos. Constr. 2020, 24, 04020010. [CrossRef] 58. Li, J.; Gravina, R.J.; Smith, S.T.; Visintin, P. Bond strength and bond stress-slip analysis of FRP bar to concrete incorporating environmental durability. Constr. Build. Mater. 2020, 261, 119860. [CrossRef] 59. Khanfour, M.; El Refai, A. Effect of freeze-thaw cycles on concrete reinforced with basalt-fiber reinforced polymer (BFRP) bars. Constr. Build. Mater. 2017, 145, 135–146. [CrossRef] 60. Shen, D.; Wen, C.; Zhu, P.; Li, M.; Ojha, B.; Li, C. Bond behavior between basalt fiber-reinforced polymer bars and concrete under cycle loading. Constr. Build. Mater. 2020, 258, 119518. [CrossRef]